Pressure-temperature phase diagram of 24 K-transition of anion ordering of(TMTSF)(2)ClO4

The anion ordering of (TMTSF)(2)ClO4 as a function of pressure was examined up to 1.1 GPa by detecting the anomaly in the temperature dependence of re sistance under pressure. It turns out that the anion ordering transition is observed up to at least 1.1 GPa by means of an extremely careful control o f the rate of slow cooling (< 5 K/h). It is remarkable that the temperature of anion ordering is found to increase up to 41 K, while the strength of t he transition observed by the magnitude of resistivity anomaly is reduced.